Mutual exclusion occurs in a Chlorella-like green alga inoculated with two viruses.

Progeny viruses resulting from dual inoculations with different and near-isogenic viruses of a Chlorella-like green alga were distinguished by immunoblotting. Plaques arising from single cells inoculated with two viruses usually contained only one of the viruses. Thus the viruses mutually exclude one another. In some combinations the ratio of viruses (as infective centres) recovered differed significantly from the input ratio.